1. Field of the Invention
This invention relates to displacement measuring instruments, and more particularly to improvements in a displacement measuring instrument wherein said instrument comprises a main scale and a carriage movable along said main scale having thereon mounted an index scale whose graduated surface is opposed through a minute space to a graduated surface of said main scale, and a value of relative displacement between the main scale and the carriage is measured from a variation in volume of a light emitted to the main scale and the index scale and transmitted therethrough or reflected thereat due to the relative movement between the scales.
2. DISCRIPTION OF THE PRIOR ART
In general, in a displacement measuring instrument for measuring a length or the like of a product, in the case a value of movement is measured between relatively movable bodies, for example a value of movement of a measuring element with respect to its main body, there has been known to be used a linear scale type displacement measuring instrument in which a detector including a main scale is fixed to one body and a carriage having mounted thereon an index scale is fixed to the other body, and a value of relative displacement between the main scale and the detector is photoelectrically read.
The linear scale type displacement measuring instrument as described above, such as a transmitting linear scale type displacement measuring instrument is of such an arrangement shown in FIGS. 1 through 3. Referring to the drawings, designated at 10 is a hollow cylindrical aluminum casing having a substantially rectangular hollow cross section, formed by cold drawing for example, which is fixed to one of members, relative positions of which are to be measured, 12 a transmitting type glass linear main scale, the lower end of which is received in a longitudinal groove 10a formed in the hollow casing 10, and elastically bonded to the hollow casing 10 through a rubber rod 14 and an elastomeric bonding agent 16 such as silicon rubber, 20 a detecting portion fixed to the other of the members, ralative positions of which are to be measured, 22 a carriage connected to an arm 20a formed at the lower portion of the detecting portion 20 through connecting means formed of a cantilever spring 24, one end of which is fixed on the arm 20a and the other end of which is annularly attached to a conical projection 23 for example, and movable along the main scale 12 in such a condition that the carriage is biased in the direction of abutting against the outer surface of the main scale 12 by the cantilever spring 24, 26 light emitting elements disposed on the carriage 22, 28 an index scale fixed on the carriage 22, 30 photoelectric elements disposed on the side opposite to the light emitting elements 26 disposed on the carriage 22 and receiving the light which has been emitted from the light emitting elements 26 and transmitted through a graduation 12a of the main scale 12 and a graduation of the index scale 28, 32 and 33 sliders constituting travel guide mechanisms for maintaining a predetermined value of interval between a graduated surface 12b or a side face 12c of the main scale 12, which serves as a scanning reference surface of the carriage 22, 34 a belt-like steel tape for preventing dust and the like from entering the hollow casing 10 from outside, and 36 magnets embedded in the hollow casing 10 for attracting the steel tape 34.
In the conventional transmitting linear scale type displacement measuring instrument as described above, if a relative movement occurs in a direction perpendicular to the paper surface in FIG. 1 between the hollow casing 10 fixed to one of two members movable relative to each other and the detecting portion 20 fixed to the other member, then the light emitted from the light emitting elements 26 and transmitted through the graduations of the index scale 28 and of the main scale 12 varies in transmitting volume, whereby the variation in transmitting volume is detected by the photoelectric elements 30, so that a value of relative movement between the hollow casing 10 and the detecting portion 20 can be measured.
The linear scale type displacement measuring instrument as described above has a characteristic feature of that a value of relative displacement between two objects can be measured with a high accuracy. However, with the conventional linear scale type displacement measuring instrument, the scanning reference surface of the carriage 22 has included the graduated surface 12b of the main scale 12 and the side face 12c of the main scale perpendicular to the graduated surface 12b. Consequently, in order to improve the measuring accuracy, it was necessary not only to finely finish the graduated surface 12b which has been finished finely in general, but also to finish the side face 12c of the main scale to render a high straightness thereto, whereby the side face 12c of the main scale is accurately in parallel to the graduation 12a. However, in actual practice, it is difficult to finely finish the side face 12c of the main scale 12, and, even if it would be possible, considerable increase in production cost would follow. Particularly, in the case the side face 12c of of the main scale 12 is engaged with the carriage 22 through the sliders 33 as in the example of the prior art, not only the straightness but also very low surface roughness are required for the side face 12c of the main scale, thus increasing the cost to a great extent.